Exhaust gas air fuel ratio sensor

ABSTRACT

An exhaust gas air/fuel ratio sensor is formed of a plurality of wafers of ceramic material. A pair of resistance sensing leads are situated within a titania exhaust gas sensor wafer. The operating temperature of the sensor wafer is maintained at a desired level by forming a pair of heater wafers and situating the heater wafers on either side of the titania wafer. Each heater wafer is comprised of a length of platinum heater wire embedded within an alumina ceramic wafer.

United States Patent [191 Beaudoin et al.

[4 1 Oct. 7, 1975 1 EXHAUST GAS AIR FUEL RATIO SENSOR [73] Assignee:Ford Motor Company, Dearborn,

Mich.

221 Filed: Jan. 15, 1975 21 Appl. No.: 541,365

[52] U.S. Cl. 338/34; 23/254 E; 73/27 R [51] Int. Cl. H01C 13/00 [58]Field of Search 338/34; 23/254 E, 255 E,

23/288 F, 288 EC; 340/237 R, 237 P; 200/6103; 252/477 R Hardtl 338/34Taguchi 338/34 Primary ExaminerC. L. Albritton Attorney, Agent, orFirmRobert A. Benziger; Keith L. Zerschling [5 7] ABSTRACT An exhaustgas air/fuel ratio sensor is formed of a plurality of wafers of ceramicmaterial. A pair of resistance sensing leads are situated within atitania exhaust gas sensor wafer. The operating temperature of thesensor wafer is maintained at a desired level by forming a pair ofheater wafers and situating the heater wafers on either side of thetitania wafer. Each heater wafer is comprised of a length of platinumheater wire embedded within an alumina ceramic wa- 4 Claims, 2 DrawingFigures References Cited fer.

UNITED STATES PATENTS 2,806,991 9/1957 White 338/34 X POWZIQ JZ/PPZ) Z6/2 xii/V301? [([CTKO lV/C j 0 MfH/VS EXHAUST GAS AIR FUEL RATIO SENSORCROSS-REFERENCE TO RELATED APPLICATIONS The present invention is animprovement to the inventions disclosed and claimed in copending,commonly assigned patent applications Ser. Nos. 391,424 and 393,698filed in the names of H. L. Stadler et al. Co-pending commonly assignedpatent application Ser. No. 375,993 filed in the names of Kushida et al.entitled Circuit for Converting a Temperature Dependent Input Signal toa Temperature Independent Output Signal is a related patent application.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention is directed to the field of exhaust gas sensors and moreparticularly to that portion of the above-noted field which is concernedwith the analysis of the exhaust gases of an internal combustion engineto generate electrical signals indicative of the air/fuel ratio of thecombustion mixture. More particularly still, the present invention isdirected to that portion of the above-noted field which is concernedwith devices for sensing exhaust chemistry, which devices rely upon theresistance change of titania ceramic oxide material in response tovariations in the partial pressure of oxygen of its immediate gaseousenvironment. More particularly still, the present invention is directedto that portion of the above-noted field which is concerned withprovision of means for maintaining the temperature of the ceramic sensormaterial at a predetermined controlled level without contaminating orotherwise influencing the measured titania resistance. More particularlystill, the present invention is directed to that portion of theabove-noted field which is concerned with the provision of a device forinsertion within the exhaust system of an internal combustion engine forperforming selective chemical analysis thereof, which device may givereliable performance over extended periods of time. More particularlystill, the present invention is directed to that portion of theabovenoted field which is concerned with devices having the capabilityof providing long duration generation of signals indicative of anexhaust gas chemistry characteristic which signals may be used toprovide feed-back control of an internal combustion engine.

2. Description of the Prior Art It is known that certain ceramicmaterials exhibit a predictable, variable electrical behavior in thepres ence of variations in the partial pressure of oxygen in the gaseousenvironment of the material. One example of this electrical phenomenonis the galvanic voltage developed by a zirconia ceramic body used toseparate gaseous mixtures having differing partial pressures of oxygen.Another example is the variation in resistance of a titania ceramicmaterial maintained at elevated temperature, set forth in theabove-noted copending, commonly assigned patent applications Ser. Nos.39l,424 and 393,698.

It has been proposed that the ability to generate an electrical signalindicative of a variation in the partial pressure of oxygen may beutilized, with suitable support electronics, to provide a feed backcontrol system for an internal combustion engine. Zirconia and titaniaexhaust gas sensors, situated within an internal combustion engineexhaust system, have demonstrated large scale electrical signaltransitions for variations in the air/fuel ratio of the combustionmixture being provided to the engine as that mixture ratio is changedfrom a rich mixture to a lean mixture through a stoichiometric mixtureratio. Any combustible air/fuel mixture having a quantity of fuel inexcess of the quantity of fuel necessary for complete combustion of theavailable quantity of air is considered to be a rich mixture.Conversely, any combustible air/fuel mixture having a quantity of air inexcess of the quantity of air necessary for complete combustion of theavailable quantity of fuel is considered to be a lean mixture. Astoichiometric mixture has quantities of fuel and of air sufficient forcomplete combustion.

In working with these devices, it has become apparent that each devicehas peculiarities which render the device less than ideal. In the caseof zirconia, exposure of one surface of the zirconia sensor toatmospheric conditions is necessary in order to provide a referencepartial pressure of oxygen. In the case of the titania, the measuredresistance is a function of temperature as well as of the partialpressure of oxygen. Titania sensors therefore require additionaltechniques for maintaining the ceramic titania sensor at a predeterminedtemperature level.

According to the above-noted co-pending commonly assigned patentapplication Ser. No. 391,424 the titania wafer may be maintained at aselected temperature, for example, in the range of 600C to 900C, byfabricating the sensor from three wafers of ceramic material with aheater element contained between the first and second wafers and theresistance sensing leads contained between the second and third wafers.One problem with this approach has resulted in the fact that atemperature gradient is established within the device which gradientpasses through the resistance sensing zone. This gradient causes themaintenance of the desired temperature, which should not vary more than2 or 3 centigrade, at the resistance sensing zone to be diffi cult. Oneway to achieve closely controlled temperatures at the resistance sensingzone would be to use a thermocouple, positioned in the resistancesensing zone, to control the application of heater energy. This approachis not desirable since thermocouples are relatively expensive devicesand they add to the complexity of the resulting sensor device. It isalso an object of the present invention to provide a means formaintaining a titania exhaust gas sensor at a selected temperaturewithout the establishment of any signficiant temperature gradients andwhich may be used reliably with external electronic heater control meansto maintain the selected temperature without the use of thermocouples.

As described in the above-noted co-pending commonly assigned patentapplication Ser. No. 393,698, it is believed that one important featureof a resistive titania exhaust gas sensor used in a feedback controlsystem for an internal combustion engine resides in rapid response timeachieved through maintaining a high degree of porosity. In order toavoid the undesired temperature gradient heating previously mentioned, apair of heaters could be situated on either side of the titania sensingwafer. Such an approach would require that the rapid gas transferthrough the device be maintained. It is therefore a further and specificobject of the present invention to provide a ceramic material forencasement of a pair of heater conductors which material will beresistive to microfracturing. It is also a further and specific objectof this invention to provide such a material which may be provided witha high degree of porosity consonant with maintaining a good responsetime for an included titania exhaust gas sensor wafer. A further problemwhich can be anticipated to occur in any sensor arrangement whichprovides for a mixture of ceramic material having interface zones is toprovide such materials which will not result in contamination of theactive sensor element by migration of the materials.

across the interface zone.

The existence of the temperature gradient may be alleviated by using apair of heater wires operated in unison and placed on opposite sides ofthe resistance sensing zone. This approach presents a further problem inthat the application of resistance wire heating of the titania fromwires embedded within the titania may produce microfractures. On the onehand, these microfractures may extend into the resistance may extendinto the resistance sensing zone causing a measurable resistance changesuch as may stimulate false regulation. On the other hand, thesemicrofractures may expose the embedded heater wire to the gaseousatmosphere surrounding the sensor body. This may cause the heater wiresto burn out. In those instances where the heater wires are coupled to atemperature controller as described in co-pending commonly assignedpatent application Ser. No. 484,896, filed July 1, 1974 in the name ofLawrence R. Foote and titled Electrical Control System for an ExhaustGas Sensor, false temperature signals may result even if burn out doesnot occur. This will cause the sensor output signal to drift from theoutput signal which should be generated under the existing conditions.It is therefore an object of this invention to provide a ceramicelement, for inclusion with the exhaust system of an internal combustionengine, to include a titania exhaust gas chemistry responsive element,which need not include temperature sensing means in the resistancesensing zone. It is a further object of the present invention to providesuch a device which is resistant to microfracture induced resistancechanges in the sensing zone. It is therefore a further and specificobject of the present invention to provide an exhaust gas air/fuel ratiosensor having an active section comprised of a variably resistivetitania ceramic material which includes a pair of resistance sensingleads and having a pair of heater wires embedded within alumina ceramicmaterial disposed on opposite sides of the active section to maintainthe active section at an operating temperature without requiringthermocouple control techniques and which will not result in any ceramiccontamination or degradation of the active titania section of the sensorand which will not provide any substantial masking of the active titaniasection to changes in the partial pressure of oxygen in the environmentof the sensor.

SUMMARY OF THE PRESENT INVENTION The present invention provides amulti-layered ceramic exhaust gas sensor body having an active variablyresistive titania section sandwiched between a pair of heater sections.The active titania section is comprised of a pair of resistance sensingleads disposed within a wafer or disc-like body of titania ceramicmaterial. Each of the heater sections is comprised of a length of heaterwire embedded within a ceramic wafer comprised essentially of alumina.The two heater sections and the active titania section are formed fromgreen ceramic material and are co-fired into a unitary ceramic body. Theheater sections are. connected electrically in series and may becommunicated to, for example, an electronic heater controlcircuitresponsive to the heater resistance for maintainingthe currentflow through the heater wires at a level whichwill provide formaintenance of the heater sections and the active titania sectionsandwiched therebetween at a selected temperature level.

Each of the three sections of the exhaust gas sensor may be comprised ofa pair of wafers or discs of the selected ceramic material, in a greenor un-fired state. The necessary electrical leads and heater wires maybe placed between the wafers while the ceramics are in a green state andthe wafers may be stacked together prior to final firing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates an electricalschematic block diagram for an exhaust gas sensor and its associatedcontrol.

FIG. .2 illustrate an exhaust gas sensor fabricated in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to thedrawing wherein like numbers designate like structure throughout thevarious views thereof, FIG. 1 illustrates an exhaust gas sensor body 10according to the present invention having an associated sensorelectronic means 12. Exhaust gas sensor body 10 is illustrated as havingresistance heater means 14 and a pair of resistance sensing leads l6, l8embedded therein. Electrical heater means 14 are energized from a powersupply 20 through leads 22, 24. Resistance sensing leads l6, 18 areconnected to the sensor electronic means 12 which also receives energyfrom the power supply 20 over conductors 26, 28. This structure isgenerally as described in the aforementioned copending commonly assignedpatent application Ser. No. 391,424 and further description is notbelieved to be necessary to an understanding of the instant invention.

Power supply 20 will provide electrical energy through the electricalheater conductor 14 to apply electric heating energy to the exhaust gassensor 10. Heater conductor 14 may be, for example, a noble metalconductor such as platinum. similarly, resistance sensing leads l6, 18would be formed of a noble metal since such material readily withstandthe temperature of ceramic firing. Power supply 20 may be, for example,the electronic heater control of co-pending commonly assigned patentapplication Ser. No. 484,896. Conversely, it could also be a simplebattery with a thermocouple control arrangement.

When resistance sensing probes l6, 18 are mutually embedded inspaced-apart relation within a titania sensor section, the electricalresistance of the titania between sensing leads 16, 18 will be afunction of a) the operating temperature and b) the partial pressure ofoxygen in the environment in which sensor body 10 is immersed. Withpower supply 20 operated to maintain the temperature generated byelectrical heating means 14 at a constant value, variations in theelectrical resistance measured by sensing probes l6, 18 will be solely afunction of the partial pressure of oxygen in the environment in whichsensor is located. The resistance sensing zone in the body of materialof the discs 38 of wafer 36 which lies between resistance sensing leadsl6, l8 and is here denoted by numeral 42. Sensor electronic means 12will receive an electrical energy over conductors 26, 28 and willmeasure the amount of resistance sensed by sense leads l6, 18 togenerate an output signal on conductor 30 which signal may be madereadily indicative of the air/fuel ratio of the combustible mixturegenerating the exhaust gas environment of sensor 10. The electricalcharacter of the signal appearing at output 30 will be a function of thetype of control desired in the feedback system in which the presentinvention may be utilized and the sensor electronic means may be readilyadapted to generate the proper kind of electrical signal.

Referring. now to FIG. 2, the exhaust gas sensor body 10 according tothe present invention is illustrated in an exploded view. Exhaust gassensor body 10 is made up of a sandwich of heater wafer means 32, 34which are arranged to be on opposite sides of a central active titaniaexhaust gas sensing section 36 is illustrated as being comprised of apair of discs 38 of titania ceramic material generally as described inthe above-noted copending commonly assigned patent applications. A pairof resistance sensing leads I6, 18 are maintained in spaced-apartrelation between discs 38. The heater wafer sections 32, 34 arecomprised of a pair of discs 40 of ceramic material, the preferredcomposition of which is described hereinbelow. A heater conductor 14 issituated between each of the discs 40 of each of the heating wafer means32, 34 and are shown to be connected electrically in series.

In fabricating the exhaust gas sensor 10 of the instant invention, theactive titania wafer 36 may be fabricated generally in accordance withthe teachings of the above-noted co-pending commonly assigned patent applications Ser. Nos. 391,424 and 393,698. A pair of resistance sensingprobe leads 16, 18 would be sandwiched between a pair of green ceramicdiscs 38 formed from a titania material as noted above and the discs 38would be uniaxially compressed to provide for an initial bondingtherebetween. According to the present invention, discs 40 are formed ofalumina ceramic material. Each of the heater wafer sections 32, 34 wouldbe fabricated by sandwiching a heater conductor 14 between a pair ofdiscs 40 which would be compressed uniaxially to provide for initialbonding. The active wafer 36 would then be sandwiched between a pair ofheater wafer means 32, 34 with further uniaxial pressure applied toprovide for initial bonding between the green ceramic material formingeach of the heater wafer sections 32, 34 and the green ceramic materialforming the active wafer 36. Thereafter, the composite would be fired toproduce a finished ceramic body. The heater wafers 32, 34 are formedadvantageously of alumina ceramic for several reasons. The coefficientof thermal expansion of the alumina ceramic more closely matches thecoefficient of thermal expansion of the platinum noble metal conductorsused as the electrical heating means 14. The alumina is sufficientlyhighly porous at thicknesses consistent with internal heater temperature control to permit rapid gas transfer from the surroundingatmosphere to the titania sensor wafer 36. The alumina also forms astrong ceramic material which aids in adding to the strength of thesensor body 10. In addition, the alumina heater wafers 32, 34, whencofired with the titania sensor wafer 36 will form a strong ceramic bondwith the titania at the interface of a ceramic disc 40 with a ceramicdisc 38.

Each of the discs 38, 40 illustrated in the FIG. 2 embodiment may be forexample one quarter of an inch in diameter and 0.025 in (twenty fivethousandths of an inch) thick prior to final firing. Resistance sensingleads 16, 18 may be fabricated from platinum conductive wire andpositioned in parallel approximately an eighth of an inch apart on thesurface of one of the wafers 38. Similarly, the heater wire 14 may befabricated from platinum resistance wire with a substantial lengththereof deposited on the surfaces of the wafers 40 to provide asufficient length to generate the necessary level of heating.

In the accomplishment of the objectives of the present invention, thewafers 40 are fabricated from an ultra fine alumina powder having a meanparticle size of less than 1.0 microns. Use of the fine particle sizealumina powder permits co-firing of the sensor assembly in order toprovide for a strong ceramic bond formation between the active sensorwafer and each of the heater wafer sections 32, 34. The alumina ceramicmaterial is comprised essentially of alumina having a chemicalcomposition A1 0 and a purity of about 95 percent. It may include minorquantities of various metal oxides. In fabricating wafers 40, a mixtureof approximately ceramic powder and 20% plastic ma terial such aspolyvinyl butyral and dibutyl phthalate may be used. This material isdeposited on a sheet and is controlled to have a thickness ofapproximately 0.025 in. The wafers are thereafter cut from the sheetwhile in a plastic state and the sensor 10 is fabricated by firstforming the heater wafers 32, 34 and then plac ing an active sensorwafer 36 therebetween. The composite is thereafter uniaxially compressedto establish initial bonds between the discs 38 and their contactingdiscs 40. The composite of green ceramic discs is thereafter fired toform a finished ceramic. The final firing may follow the timetemperature schedule set forth in the above-mentioned patentapplications Ser. Nos. 391,424 and 393,698. The ceramist will recognizethat other time-temperature relationships could be followed to arrive ata finished sensor body 10. The resulting heater wafer portions 32, 34 ofthe sensor body 10 have a typical pore size (diameter) of 0.15 micronswith about 65% of the pore volume being comprised of pores having a sizeof from about 0. 10 microns to about 0.20 microns.

It will be seen that the present invention readily accomplishes itsstated objectives. By using alumina ceramic material to provide theceramic portion of each of the heater wafers 32, 34, a strong ceramicmaterial is provided to protect the heater elements with the ceramicmaterial being fully compatible with the titania material whileresisting any tendency to produce ceramic material migration orcontamination. By use of the fine particle size a relatively porousceramic body is produced so that, when considering the overall thicknessbeing in the neighborhood of fifty thousandths of an inch, the transporttime for gas to penetrate through the heater wafers 32, 34 of the sensorbody 10 to arrive at the active sensing wafer 36 is sufficiently lowthat good response time for the sensor body 10 can be obtained.Furthermore, the alumina and titania are sufficiently compatible so thatstrong ceramic .bonds are formed between the mating interface surfacesof the heater wafers 32, 34 and the active wafer 36. By providing twoheater wafers disposed on opposite sides of the active section, thegeneration of undesirable temperature gradients through the acitvesection such as each of said heater wafers comprising electricalresistance heater means embedded within an alumina ceramic wafer.

2. The sensor of claim 1 wherein each of said heater wafers is comprisedof alumina ceramic forming powders having a purity of at least 3. Thesensor of claim 2 wherein said alumina ceramic forming powders have amean particle size of less than about 1.0 microns.

4. The sensor of claim 1 wherein each of said heater wafers comprises apair of discs of green alumina ceramic material having the electricalresistance heater means sandwiched therebetween with the discs maturedby cofiring with the titania wafer.

1. In combination with a resistive type exhaust gas sensor of the typehaving a pair of resistance sensing leads embedded within a titaniaceramic wafer having a pair of surfaces, the improvement comprising: apair of heater wafers disposed in contact with, and forming ceramicbonds with, the surfaces of the titania wafer; each of said heaterwafers comprising electrical resistance heater means embedded within analumina ceramic wafer.
 2. The sensor of claim 1 wherein each of saidheater wafers is comprised of alumina ceramic forming powders having apurity of at least 95%.
 3. The sensor of claim 2 wherein said aluminaceramic forming powders have a mean particle size of less than about 1.0microns.
 4. The sensor of claim 1 wherein each of said heater waferscomprises a pair of discs of green alumina ceramic material having theelectrical resistance heater means sandwiched therebetween with thediscs matured by cofiring with the titania wafer.